Cross-point contact-free floating-gate memory array with silicided buried bitlines
RELATED PATENT APPLICATIONS 
     This application discloses subject matter also disclosed in co-pending U.S. patent application Nos. 07/269,837; 07/269,849; 07/269,836 and 07/270,594, all of which are hereby incorporated herein and filed herewith and all of which are also assigned to Texas Instruments Incorporated. 
     BACKGROUND OF THE INVENTION 
     This invention relates to integrated-circuit erasable-programmable-read-only-memory (EPROM) arrays and to electrically-erasable-programmable-read-only-memory (EEPROM) arrays and, in particular, to EPROM and EEPROM structures having buried bitlines that include source-drain regions. 
     Floating-gate, avalanche-injection, metal-oxide-semiconductor (FAMOS) structures having buried, diffused bitlines that include source-drain regions are described in U.S. Pat. No. 4,373,248 issued Feb. 15, 1983 to David J. McElroy and in U.S. patent application No. 128,549 filed Dec. 3, 1987, both assigned to Texas Instruments Incorporated. Such structures have several advantages over other structures, including smaller memory-cell area, reduced number of contacts, and planar array topography. On the other hand, the described structures have disadvantages, including an interdependent relationship between N+ junction depth and the thickness of the isolation oxide over the buried bitlines, high bitline junction capacitance, high drain-to-floating-gate capacitance and non-silicided bitlines. In addition, the described structures cannot be linearly shrunk without decreasing the coupling ratio of the control-gate voltage to the floating-gate voltage during programming and/or erasing. 
     The capacitance of prior-art buried N+ bitlines is high because of the required deeply buried N+ junction that assures adequate grown oxide thickness above the buried N+, because of the required relatively wide bitlines that ensure adequate coupling between polycrystalline silicon (polysilicon) layers, and because of the junction capacitance in the bitline isolation region. See, for example, J. Esquivel, et al.; IEDM Tech. Digest Papers, 592 (1986). 
     Prior-art buried N+ FAMOS devices have high drain-to-floating-gate capacitive coupling, leading to a small margin between the device drain-coupled turn-on voltage and the device breakdown voltage during programming. As a result, a part of the programming current may flow through non-selected bits sharing a bitline with the selected bit, causing reduced threshold voltage shift in the programmed bits. 
     The buried bitlines of prior-art devices generally cannot be silicided because the buried N+ junctions and the oxide insulating regions over those junctions are formed early in the process, before formation of the gate oxide, the floating gate, the control or programming gate and other field-effect devices. Some of the later-occurring steps in the process for forming those elements require temperatures higher than 900° Celsius, and processes at temperatures that high are not desirable after siliciding of the buried N+ bitlines. Without siliciding, the bitlines have a high resistance that requires space-consuming connections with parallel metal conductors at intervals of every few transistor cells. A prior-art buried bitline process and structure is described in U.S. Pat. No. 4,597,060 issued June 4, 1986 to Allan T. Mitchell and assigned to Texas Instruments Incorporated. The process and structure of that invention requires formation of the silicided bitlines prior to formation of the inter-level dielectric layer and combined control gates/wordlines. 
     Accordingly, there is a need for an integrated-circuit structure and a procedure for fabricating that structure such that bitline and drain-to-floating-gate capacitances are decreased, such that the N+ junction depth/profile can be optimized independent of oxide thickness above the bitlines, and such that the coupling ratio of the control-gate voltage to the floating-gate voltage does not decrease as the memory cell dimensions are downsized. In addition, there is a need for bitlines that are silicided to allow elimination of presently used metal interconnect conductors above and parallel to buried bitlines. 
     SUMMARY OF THE INVENTION 
     The buried-bitline structure of this invention decreases bitline and drain-to-floating-gate capacitances and permits downsizing of memory-cell dimensions without adversely affecting the coupling ratio of the control-gate voltage to the floating-gate voltage during programming or erasing. The bitline capacitance is reduced through use of NSAG (N+ Self-Aligned Gate) bitlines that are buried under wordlines. Reduced total lateral diffusion of the NSAG bitlines decreases the capacitance between the drain and the floating-gate. The process permits the bitlines to be silicided to lower the bitline resistance an therefore to decrease the number of contacts necessary, eliminating the need for parallel interconnect conductors. The N+ junction depth/profile is independent of the thickness of the oxide above the buried bitlines. Bitline isolation is by thick oxide, rather than by P/N junction or by trench isolation as in co-pending U.S. patent application No. 07/269,837, also assigned to Texas Instruments Incorporated. Bitlines are not paired with common ground conductors as in co-pending U.S. patent application No. 07/269,849, also assigned to Texas Instruments Incorporated. The area required for each cell is less than that required by prior-art cells. In addition, coupling ratio of the control-gate-to-floating-gate capacitance and the floating-gate-to-substrate capacitance is a function of overlap of the control and floating gates on the thick field oxide as well as of the dielectric properties and thickness of the insulating layers between those elements. 
     The structure and process described herein require less stringent mask alignments than those required for the structure and process described in U.S. patent application No. 07/270,594 filed herewith. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The novel features of this invention are set forth in the appended claims. The invention, its features, and its advantages are described below in conjunction with the following drawings: 
     FIG. 1 represents an isometric view, in section, of a part of the cross-point FAMOS array of this invention. The two center sections represent elevation views drawn through the gates of the transistors of the embodiments. The two outside sections represent elevation views drawn through regions between those gates. 
     FIG. 2 represents a top view of the device of this invention. 
     FIGS. 3a-3d represent, in section as indicated, elevation views of the device of FIG. 2 and of the right inner section of FIG. 1 at various stages of fabrication. 
     FIGS. 4a-4d represent, in section as indicated, elevation views of the device of FIG. 2 and of the left inner section of FIG. 1 at various stages of fabrication. 
    
    
     DETAILED DESCRIPTION OF THE SPECIFIC EMBODIMENT 
     Referring to FIG. 1, for example, FAMOS transistors 1 include a plurality of buried conductors 2 in the face of semiconductor substrate 3. Buried conductors 2 are formed using a NSAG process and include silicided upper-surface regions 4. The conductors 2 are heavily doped regions buried under a relatively thick first insulating oxide strips 5, and are continuous through the array of FAMOS transistors. Alternate buried conductors 2 are bitlines. Buried conductors 2 are isolated from each other in part by thick field oxide regions 6, which are illustrated in FIG. 1 as grown field oxide regions 6 over P-type channel-stop regions 7. Conductors 2 include N+ doped source-drain regions SD adjacent each channel region C. Each polysilicon floating gate 8 is separated from a channel region C by gate oxide layer 9 and from a control gate section 10 by an inter-level dielectric layer 11. Floating gate 8, inter-level dielectric layer 11 and control gate section 10 extend over field oxide regions 6 on the non-source-drain sides of the cell. For use in EEPROM applications and as illustrated in FIGS. 3b-3d, gate oxide layer 9 may have a thin tunnelling region 9 a for erasing. Tunnelling region 9a may have a thickness of perhaps 100 Angstroms, as compared to 350-400 Angstroms for the remainder of gate oxide layer 9. Silicided bitlines 2 include N+ doped source-drain regions SD. Sidewall oxide spacers 12 may be used to separate silicided regions 4 from floating gate 8 and are located adjacent the sides of each floating gate 8 and of each control gate 10 above each source-drain region SD. Wordlines 13, which may be silicided polysilicon or refractory metal, connect control gates 10. Wordlines 13 are substantially at right angles with respect to conductors 2. Second insulating oxide strips 14 are located between parallel wordlines 13, separating gates 8 and 10, and are above field oxide regions 6 and first insulating oxide strips 5. 
     The extensions of floating gate 8, inter-level dielectric layer 11 and control gate 10 over the upper surface of field oxide region 6 improve the coupling of programming and erasing voltages to floating gate 8. The material for and dimensions of inter-level dielectric layer 11 and gate oxide layer 9 may be chosen to improve further the coupling to the floating gate of programming and erasing signals applied at control gate 10. As is well-known, the capacitance between control gate 10 and floating gate 8 should ideally be much greater than the capacitance between floating gate 8 and other nodes for appropriate coupling of the programming voltage to produce either tunnelling of electrons between the substrate 3 and floating gate 8 in EEPROM applications or avalanche injection to the floating gate 8 in EPROM applications. And as in all floating-gate, nonvolatile memory devices, those capacitances vary according to relative sizes of gate surfaces and channel surfaces, according to dielectric types, and according to dielectric thicknesses. 
     It is desirable for memory-cell arrays to have a very low bitline capacitance because a small bitline capacitance improves speed of operation. In prior-art buried conductor cells the bitline capacitance is determined by the depth and area of the depletion region formed by the reverse-biased diode junction of what would correspond to the N+ conductor 2 and the P-doped substrate 3 of this invention. The bitline capacitance of the device of this invention is less than that of prior-art structures because the area of N+/P junction is reduced by narrowing bitline and by reducing the N+/P junction depth. 
     As illustrated in the embodiment of FIG. 1, the overlap of floating gate 8 on field oxide 6 increases the capacitance between control gate 10 and floating gate 8, thereby increasing the coupling of programming voltages to floating gate 8. In addition, judicious choices of thickness and of dielectric constants for gate oxide layer 9 and inter-level dielectric layer 11 may be made to increase that coupling. In general, the thickness of inter-level dielectric layer 11 should be less than that of gate oxide layer 9 and the dielectric constant of inter-level dielectric layer 11 should be greater than that of gate oxide layer 9 for improved coupling. 
     A method of making the device of FIG. 1 will be described in reference to FIGS. 3a-3d and 4a-4d. The starting material is a slice of P-type silicon, of which the semiconductor substrate 3 is a very small portion. The slice is perhaps six inches in diameter, while the portion shown in FIG. 1 is only a few microns wide. A number of process steps would be performed to create transistors peripheral to the array, and these steps are not discussed herein. For example, the memory device would typically be of the field-effect type having N-wells and P-wells formed in the substrate. Those N-wells and P-wells may require a voltage-threshold adjustment process, as is well-known. The first step related to the cell array of the invention is the forming of thick field oxide regions 6 and channel stop regions 7 using a well-known LOCOS or high pressure oxidation process. Referring now to FIGS. 2 and 4a, regions 6 and 7 are formed in the parts of the surface of substrate 3 marked P in FIG. 2 where field oxide 6 is to be grown. As part of the LOCOS or high pressure oxidation procedure, areas of substrate 3 not covered by oxide/nitride layer 15 are subjected to a boron impurity implant 16, as illustrated in FIG. 4a. The substrate is then subjected to an oxidizing atmosphere, in accordance with well-known procedures, to form regions 6 and 7 as illustrated in FIG. 4b. 
     As indicated in FIG. 3a, the next step related to the cell array of the invention is forming a gate oxide layer 9 approximately 350 Angstroms thick by exposing the face of substrate 3 to oxygen and gaseous hydrochloric acid at ambient pressure and at a temperature of approximately 950° Celsius for about 50 minutes, using conventional gate oxidation procedure. Tunnelling region 9a may be formed by patterning and etching gate oxide layer 9, re-growing oxide if necessary. 
     Next, first conductive or polysilicon floating gate 8 layer is applied to the surface, as also indicated in FIG. 3a. The first conductive or floating gate 8 layer may be about 3000 Angstroms thick and is doped N+. 
     Referring again to FIG. 3a, inter-level dielectric layer 11 is then formed of silicon oxide, or of oxide-nitride-oxide, or of aluminum oxide, or of aluminum nitride, or of tantalum pentaoxide, for example, according to well-known procedures. The dielectric constant of inter-level dielectric layer 11 should be as large as possible and its thickness should be small as possible, consistent with EPROM and EEPROM requirements for wordline-to-floating gate capacitive coupling ratio, for dielectric breakdown voltage, and for dielectric charge leakage criteria. 
     Next, second conductive or polysilicon control gate 10 layer is applied to the surface using conventional procedures, as also indicated in FIG. 3a. Second conductive or control gate 10 layer may also be approximately 3000 Angstroms thick and is doped N+. 
     A protective layer 17 of oxide, or of oxide-nitride, is then deposited over the surface using well-known methods, as illustrated in FIG. 3a. 
     Referring now to FIG. 3b, strips that will become control gates 10 and floating gates 8 are formed by patterning the surface of protective layer 17 and stack-etching through protective layer 17, control gate 10 layer, inter-level dielectric layer 11 and floating gate 8 layer. An oxide may then be grown on the sides of the strips that will become control gates 10 and floating gate 8 for the purpose of improving data retention in floating gates 8. 
     As illustrated in FIG. 3b, conductors 2 are then formed by implanting with arsenic to create an abrupt junction on the programming side and by double implantation of both arsenic and phosphorus to create a graded junction on the read side, indicated as N+ source-drain regions SD in the surface of substrate 3. The implantation is followed by a standard implant anneal at appropriate temperature. The arsenic/phosphorus implant provides a graded junction that avoids read disturb caused by hot electron effect and that suppresses write disturb during programming. 
     As also illustrated in FIG. 3b, sidewall oxide regions 12 may then be formed on the sides of the strips that will become control gates 10 and floating gates 8 using well-known procedures, such as those described in U.S. Pat. No. 4,566,175, issued Jan. 28, 1986 and assigned to Texas Instruments Incorporated. 
     Alternatively, a lightly doped (commonly referred to as LDD) junction profile may be formed on the read side to avoid read/write disturb by performing the arsenic/phosphorus implant and annealing step after sidewall oxide regions 12 are formed, as also described in U.S. Pat. No. 4,566,175 referenced above. Whether the junction is lightly doped or doubly diffused, it should completely underlay tunnelling region 9a to avoid read disturb. 
     The conductors 2 are then silicided in accordance with well-known procedures to form silicide regions 4. One such procedure is described in U.S. Pat. No. 4,672,419, issued June 9, 1987, and assigned to Texas Instruments Incorporated. It is desirable to select subsequent processes that do not require temperatures exceeding 900° Celsius to prevent dopant diffusion from the junction to the silicided region, and to avoid junction spiking. 
     Referring now to FIG. 3c, the structure is then made planar by using a procedure that requires a temperature of 900° Celsius or less. One such procedure is the deposition of a thick oxide over the structure and the use of a resist etch-back process to remove the higher elevations of that oxide. The etch removes protective layer 17 from the upper surfaces of strips that will become control gates 10 and floating gates 8. As a result of this step, bitlines 2 are buried under first insulating oxide strips 5. 
     Referring now to FIGS. 3d and 4d, a third conductive or wordline 13 polysilicon layer is formed on the surface of the structure and is doped N+. Third conductive or polysilicon wordline 13 layer may be silicided for increased conductivity. Wordline 13 layer may also be formed from layered refractory metal and polysilicon. In the alternative, wordline 13 layer may be formed from a refractory metal such as tungsten, or from a refractory silicide such as tungsten silicide or titanium silicide. Wordlines 13 are patterned and stack etching is conducted through wordline 13 layer, control gate 10 strips, inter-level dielectric layers 11 and floating gate 8 strips to form wordlines 13, control gates 10 and floating gates 8, as illustrated in FIGS. 1, 3d and 4d. The generally practiced plasma etch technique chosen for this step should etch polysilicon faster than oxide in order that a relatively small part of first oxide insulating strips 5 and sidewall oxide regions 12 will be removed. Again, an oxide coating may be formed on the sides of control gates 10 and floating gates 8 to improve data retention in floating gates 8. Also as illustrated in FIG. 4d, the structure may again be made planar by depositing an oxide and using a resist etch-back procedure similar to that used previously to form second insulating oxide strips 14. 
     The upper surface may then be covered with a final layer of oxide, again made planar, and etched at appropriate locations to provide contact with metal conductors, which are formed by depositing a metal layer, then patterning and etching that metal layer. 
     Second insulating oxide strips 14 may be discontinuous at points where those strips 14 pass over thick field oxide regions 6, depending on the relative heights of elements and on the processes used. 
     While this invention has been described with respect to an illustrative embodiment, this description is not intended to be construed in a limiting sense. Upon reference to this description, various modifications of the illustrative embodiment, as well as other embodiments of the invention, will be apparent to persons skilled in the art. It is contemplated that the appended claims will cover any such modifications or embodiments that fall within the scope of the invention.